Cable sizing

[Happy1]

Using a web calc recommended on forum inputting
12v, 5amp, 8m length it shows 16mm result

On eBay 0.5mm (2x16/0.2) 11 amp
http://www.ebay.co.uk/itm/AUTOMOTIV...hash=item2f05378c37:m:m543JreWmyA2crRz8mcBVnw
Is this right cable size or am I horribly wrong?

 

[maxi77]

Both are in their own way right. In selecting cable sizes we have to problems to deal with, first the absolute current capacity of the cable this is the smallest you should use what ever, overloading it can cause fire so it needs to be fused for protection. then there is the problem of voltage drop. quite how you deal with this depends on quite what you are powering. On boats I was very careful sizing the cable for navigation lights as voltage drop would stop them giving their rated output so those circuits had relatively large cables. On the other hand cigar sockets close to the battery did not need special treatment. Voltage drop is a factor of of cable size, current and cable length. Theses sites may help in both understanding and making your choice

http://www.12voltplanet.co.uk/cable-sizing-selection.html

 

[Robin McHood]

Size for a 8m run should be 4mm based on 12v and 5Amp. This will give a drop of almost 0.28v over that length. The smaller you want that voltage drop to be the bigger the cross section of the cable required. A 3% drop is usually acceptable. At low voltages its the voltage drop that is the decider, sure a 0.5mm cable will carry 5A but at 12vDC you would lose almost 3 volts.

 

[Peter A Forbes]

Remember also that in the event of a short circuit, the cable has to be able to sustain sufficient current to blow the fuse feeding it, otherwise the cable will overheat and possibly catch fire.

12V at 5A over 5metres I would use 6sq mm.

Peter

 

[Happy1]

thanks all, guessed that the overall rating had to be sufficient to cover max amps but didn't realise length had such an impact on size.
the only confusing thing in the calculator example is the 16mm result in relation to the cable size of 0.5mm , presume it relates to figures in bracket for cable supplier?
not a 16mm diameter cable!!

^{_{Subscribers  do not see these advertisements}}

 

[Peter A Forbes]

16sq mm is the conductor cross-sectional area, which is denoted by 'sq mm' after the number.

Peter

 

[Gizmouk]

@maxi77 gives good information in his post, so I wont repeat.

Where you see the stated size of a cable, it's referring to the Cross Sectional Area (CSA). Therefore 16mm won't be 16mm diameter, but approx. 5mm
The actual physical diameter will vary depending on conductor type(s) and number, and the type of insulation material(s).

Personally, I would go for minimum volt drop across the circuit - especially given that we only have (approx) 12V to start with

 

[pappajohn]

(2x16/0.2)

That figure means 2 wires each consisting of 16 strands of 0.2mm cross sectional wire.
It could be any reasonable number of strands with a corresponding size wire.
More strands for the same size wire means more flexible.

 

[maxi77]

Size for a 8m run should be 4mm based on 12v and 5Amp. This will give a drop of almost 0.28v over that length. The smaller you want that voltage drop to be the bigger the cross section of the cable required. A 3% drop is usually acceptable. At low voltages its the voltage drop that is the decider, sure a 0.5mm cable will carry 5A but at 12vDC you would lose almost 3 volts.

It all depends on what you are powering some things are very voltage dependant whilst others are equally tolerant of voltage variation, on a fan for example few would notice any difference between a 3% drop and a 6% drop but 10% might start to show, on the other hand many electronic devices with a nominal 12 volt supply will tolerate voltages between 10 and 20 volts say some more, some less. The 16mm cables recommendation gives a voltage drop of less than 1% which is in the ball park I used to aim for with boat navigation lights which were voltage sensitive in reaching their required output

 

[ymfb]

I'm not an electrician but I'd be surprised if there are many cables bigger than 2.5mm2 other than the starter motor on a MoHo. The cable lengths are fairly short so is voltage drop an issue.

^{_{Subscribers  do not see these advertisements}}

 

[Gizmouk]

I'm not an electrician but I'd be surprised if there are many cables bigger than 2.5mm2 other than the starter motor on a MoHo. The cable lengths are fairly short so is voltage drop an issue.

We sometimes use 630mm singles, but try to avoid anything more than 300mm due to bend radius and cost of crimps. Obviously not on MHs though

 

[Peter A Forbes]

I'm not an electrician but I'd be surprised if there are many cables bigger than 2.5mm2 other than the starter motor on a MoHo. The cable lengths are fairly short so is voltage drop an issue.

Alternator on our V8 Land Rovers is probably 35sq mm, but generally all cabling is thinner for individually fused circuits.

Peter

 

[Happy1]

thanks guys
the .5mm looks like a strand of hair in comparison to the 2.0mm i ordered at the same time, negligible difference in cost per metre